LEDs are current-controlled devices in the sense that the intensity of the light emitted from an LED is related to the amount of current driven through the LED. FIG. 1 shows a typical relationship of relative luminosity to forward current in an LED. The longevity or useful life of LEDs is specified in terms of acceptable long-term light output degradation. Light output degradation of LEDs is primarily a function of current density over the elapsed on-time period. LEDs driven at higher levels of forward current will degrade faster, and therefore have a shorter useful life, than the same LEDs driven at lower levels of forward current. It therefore is advantageous in LED lighting systems to carefully and reliably control the amount of current through the LEDs in order to achieve the desired illumination intensity while also maximizing the life of the LEDs.
LED driving circuits, and any circuit which is designed to regulate the power delivered to a load can generally be categorized as either linear or active. Both types of circuits limit either the voltage, or current (or both) delivered to the load, and regulate it over a range of changing input conditions. For example, in an automotive environment the voltage available to an LED driving circuit can range from 9V to 15 Vdc. A regulator circuit is employed to keep the current delivered to the LEDs at a relatively constant rate over this wide input range so that the LED output intensity does not noticeably vary with every fluctuation in the system voltage.
Linear regulators are one type of device or circuit commonly employed to accomplish this task. A linear regulator keeps its output in regulation only as long as the input voltage is greater than the required output voltage plus a required overhead (dropout voltage). Once the input to the regulator drops below this voltage, the regulator drops out of regulation and its output lowers in response to a lowering input. In a linear regulation circuit, the input current drawn by the circuit is the same as the output current supplied to the load (plus a negligible amount of current consumed in the regulator itself). As the input voltage presented to the linear regulator rises, the excess power delivered to the system is dissipated as heat in the regulator. When the input voltage is above the dropout threshold, the power dissipated in the regulator is directly proportional to the input voltage. For this reason, linear regulators are not very efficient circuits when the input voltage is much larger than the required output voltage. However, when this input to output difference is not too great, linear regulators can be sufficient, and are commonly used due to their simplicity, small size and low cost. Because linear regulators drop out of regulation when the input is below a certain operating threshold, they can also be employed in LED driving circuits to effect a crude dimming function in response to an input voltage which is intentionally lowered with the desire to reduce the LED intensity. The dimming is “crude” in that it is not a linear response for two reasons. First, in the upper ranges of the input voltage above the dropout threshold, the regulator will hold the output in regulation and the LEDs will not dim at all. Once the dropout threshold is reached, the output voltage will drop fairly linearly with a further drop in input. However, LEDs are not linear devices and small changes in voltage result in large changes in current which correspondingly effect large changes in output intensity. As the voltage applied to an LED is lowered below a certain threshold, no current will flow through the LED and no light will be produced. FIG. 2 is an example of a linear regulator circuit configured to drive an LED load. FIGS. 3 and 4 give an example of the response of this linear regulated LED circuit to a dimmed input voltage.
The lower power efficiency of linear regulators makes them a poor choice in large power systems and in systems where the input voltage is much larger than the required LED driving voltage. As such, these systems typically do not employ them. Additionally, because of the requirement that the input voltage be higher than the output voltage in a linear regulator, it is not a viable choice where a higher output than input voltage is needed such as a low voltage source driving a series string of LEDs. As LEDs have increased in power and luminous output, it has become common to employ driving circuits that are active, meaning the power delivered to the end system is dynamically adapted to the requirements of the load, and over changing input conditions. This results in increased system efficiency and less heat dissipated by the driving circuitry. Such active driving circuits are commonly implemented using switching regulators configured as buck, boost, or buck-boost regulators with outputs that are set to constant-voltage, or constant-current depending on the circuit. Typically, in LED driving applications, the switching regulator circuit is adapted to sense the current through the LEDs, and dynamically adjust the output so as to achieve and maintain a constant current through the LEDs. FIG. 6 depicts a typical buck regulator circuit configured to drive an LED load at a constant current.
Many switching regulator devices have been specifically designed for driving high powered LEDs. Manufacturers have built into these devices, inputs which can be pulsed with a PWM (pulse width modulation) or PFM (pulse frequency modulation) control signal or other digital pulsing methods in order to effect a lowering of the output of the switching regulator specifically designed to dim the LEDs. Some devices also have analog inputs which lower the output to the LEDs in response to an input which is lowered over an analog range. With such dimming capabilities built into the switching regulators, very accurate linear dimming of the LEDs can be achieved. Such dimming is controlled via a network, or some user interface which generates input signals that are converted to the required digital pulses or analog signals that are sent to the switching regulator driver. This method of dimming in LED lighting systems is common. However, it requires control circuitry and user interface equipment which adds a level of cost and complexity to the lighting system.
In many cases, lighting systems and wiring are already installed, and it is desired to replace these lights with LED lights. Or, it is desired to add LED lights to an existing system and have them work in harmony with lights and equipment, which are not LED based. There are common household wall dimmers which are employed to dim incandescent lights, and there are high-end theatrical dimming systems which are used to dim entire lighting installations. These types of dimmers only affect the input voltage delivered to the Lights. There is no additional control signal which is sent to them. Therefore, LED lights which are designed to work in these systems must dim in response to a change in the input voltage.
As noted above, linear regulator based LED drivers will dim in response to a lowering of the input voltage. However the dimming is very non-linear and these regulators are inefficient. Switching regulator drivers will also fall out of regulation and dim their output when the input voltage drops below a certain threshold, but as with linear regulators, when the input is above a threshold, their outputs will be held in regulation and the LED intensity will remain unchanged. And, as in linear regulation circuits, when the switcher circuit is out of regulation, the LED response to the lowering output is very non-linear.
An even greater problem with dimming switching regulator drivers by lowering their input voltage is that these circuits need a certain start-up voltage to operate. Below this voltage, the switching regulator either shuts off completely, or provides sporadic pulses to the LEDs as it attempts to start-up, or passes some leakage current to the LEDs which causes them to glow slightly and never dim to zero. In LED circuits employing multiple lights, each driver circuit can have slightly different thresholds, resulting in differing responses at low dimming ranges. As a result, some lights may flicker, some may be off and some may glow below the threshold voltage. This is unacceptable in most lighting systems that are required to dim using standard ac dimming controllers.
The Modified Dimming LED Driver patent application referenced above detailed an LED driver based on efficient switching regulators which provides smooth and linear dimming from 100% to off, in response to the dimming input voltage that is provided with industry standard ac dimmers.
However, several difficulties arise when the input source for the driver circuit detailed in the referenced application is an electronic low-voltage transformer intended for use with an incandescent bulb. Such transformers are frequently found in track lighting and other low-voltage lighting fixtures.
These difficulties lie in the nature of the load presented by an LED lamp and its driving circuit, especially in the case of a small bulb replacement LED lamp. One of the advantages of an LED lamp over an incandescent lamp is its greater efficiency in converting electric energy into light. A typical incandescent bulb produces about 14-17.5 lumens per watt, and most halogen lamps produce about 16-21 lumens per watt. In comparison, LEDs achieving 80-100 lumens per watt are now common. Even when considering the power that is lost in the driving circuitry of an LED lamp which may be 60-80% efficient, LED lamps that are three to six times as efficient as incandescent and halogen bulbs are easily achievable. Thus an LED lamp designed to replace a halogen bulb for example would draw much less power from the transformer than the halogen for which the transformer was designed. This becomes a problem for many electronic transformers which require a minimum load to operate. Typical transformers designed to drive 50 W halogen bulbs will not start up with loads less than 10-20 W. An LED bulb designed to replace such a halogen may only draw 5-10 W. In fact, since a primary design goal for such an LED replacement lamp would be to produce similar light while drawing as little power as possible, the most efficient LED lamps would have a problem with many low-voltage electronic transformers.
It is common in the industry for such LED lamps to specify that they are only guaranteed to work with magnetic transformers. Another practice sometimes involves introducing a “dummy” load in the form of a resistor either externally or internal to the LED driver circuit. Such dummy loads may satisfy the transformer, allowing it to turn on and energize the lamp; however, they sacrifice the inherent efficiency of the LED lamp, and waste energy in the form of excess heat.
Another problem with an LED lamp operating from an electronic transformer is the type of load that the lamp provides. Regular incandescents and halogen lamps produce light when current through a tungsten filament causes it to heat up and glow white hot. The filament presents a resistive load to the transformer. In a resistive load, the current drawn by the load is directly proportional to the voltage applied to the load: I=V/R where R is the resistance. As can be seen in FIG. 6, the input of a typical switching regulator circuit contains a bulk capacitor C1, which presents a capacitive load to the electronic transformer. In a capacitive load, the current is proportional to the change in voltage over time: I=C ΔV/ΔT. The faster the voltage changes, the greater the instantaneous current drawn. With a magnetic transformer supplying the input voltage, the input is a sinewave with the same 50-60 Hz frequency as the line input. In this case, the current “surge” is only great when the capacitor C1 is discharged and the power is switched on at close to the peak of the sinewave. This does not pose much of a problem with magnetic transformers which can handle the surge, and the switching regulator input circuitry can be protected from the surge with a simple added resistor or thermistor in series with the AC input. Thermistors are widely used as inrush current limiters in switching power supplies.
However, when using electronic transformers to drive capacitive loads, such as those presented by a typical switching regulator circuit, greater problems arise. This can be understood through an examination of the output waveform of an electronic transformer. As shown in FIG. 5, electronic transformers actually provide a pulsed PWM output with a 50-60 Hz sinewave envelope on the magnitude of the pulses. The frequency of the PWM output is typically 25-100 KHz. These PWM pulses present a much faster rise and fall of the input voltage (higher ΔV/ΔT) than a slow 60 Hz sinewave, causing high current spikes 25,000 to 100,000 times per second. These current surges not only stress the input components (rectifier diodes and bulk capacitors) of the switching regulator circuit, but in some cases could trigger over-current protection circuitry in the electronic transformer causing it to shut down.
For these reasons, many electronic transformers in existing incandescent and halogen lighting fixtures do not function properly with LED lamps retrofitted into the fixture. Common results include flickering, flashing, dim output illumination, or in many cases the LED lamp will not light at all. If the transformer functions and the lamp does operate, it may experience overheating of the input components and early life failure due to the input current spikes.
Even with some electronic transformers that will function with lighter loads, there is another phenomenon which presents a problem when driving an LED lamp. Most electronic transformers rely on the resistive load of an incandescent lamp in order to oscillate at their designed PWM frequency. The capacitive load typical of switching regulator circuits can cause the PWM frequency of the transformer output to shift, which in turn causes the RMS output voltage of the transformer to deviate from its designed level. This becomes a problem when the transformer is driving an LED circuit which is sensing the input RMS voltage in order to provide dimming of the LED output.
Circuits described in the Modified Dimming LED Driver patent application referenced above, are set to drive the LEDs to maximum illumination when the input voltage from the transformer is above a certain level. If the maximum input voltage of the driving transformer varies by transformer, then the dimming curve programmed into the LED driver circuit will be sub-optimal for some transformers. The LED output may not reach full intensity with some transformers that output a lower than expected voltage, and the dimming may not vary over the full possible range with transformers producing higher output voltage.
Because of the reasons discussed above, there is need in the industry for an LED lamp that overcomes the limitations of typical low-voltage electronic transformers, providing a load which is sufficient to cause such transformers to reliably energize, but which does not cause excessive current spiking, and which does not compromise the inherent efficiency of the LED bulb through wasted energy and excess heat dissipated in a “dummy” resistive load. There is also need for such an LED lamp to dim from full output to off when driven by transformers that vary their RMS output voltage in response to typical dimmers, and to be adaptable to various transformers such that the LED lamp may be retrofitted in a wide array of installed fixtures intended for incandescent lamps.
It is an object of the present invention to provide a complete LED lamp with integral dimmable driving circuitry such as that disclosed in the Modified Dimming LED Driver application referenced above, and which functions with a wide variety of previously installed electronic and magnetic low-voltage transformers designed for incandescent bulbs. It is a further object of the present invention to provide an LED lamp which sufficiently loads such electronic transformers to cause them to energize, but which does not detract significantly from the efficiency of the LED lamp through an added resistive “dummy” load, and which diminishes the problematic current spikes seen with typical capacitive loads. It is yet a further object of the present invention to provide an LED lamp with a dimmable illumination output which is maximized to the capabilities of the particular low-voltage transformer, and which adapts automatically to each transformer, providing the maximum desired LED output illumination when the particular driving transformer is providing its maximum voltage output.